1. Field
The following description relates to an organic light-emitting display device in which the reflection of external light is reduced or prevented, and more particularly, to an organic light-emitting display device including a black matrix-containing neutral density (ND) film by which the reflection of external light is substantially or fully suppressed, and light emitted from pixels is substantially or fully transmitted, and thus the organic light-emitting display device has enhanced visibility under external light.
2. Description of Related Art
Organic light-emitting display devices, which are self-emission display devices, have a wide viewing angle, high contrast ratio, quick response time, high luminance, and low driving voltage, and can also display multiple color images.
Conventional organic light-emitting devices may have a structure in which an anode is formed on a substrate, and a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL), and a cathode are sequentially formed on the anode. The HTL, the EML, and the ETL are generally formed of an organic compound, and these three layers are collectively referred to as “organic emission layer.” When a voltage is applied to an anode and a cathode, holes injected from the anode migrate to the emission layer via the HTL, and electrons injected from the cathode migrate to the emission layer via the ETL. Carriers such as holes and electrons are recombined with each other in the emission layer to generate exitons, and the excitons are converted from an excited state to a ground state to emit light.
Organic light-emitting display devices are divided into a top-emission type and a bottom-emission type according to a direction in which pixels composed of organic light-emitting devices emit light. A bottom-emission type organic light-emitting display device has a light-emitting region and a non-light emitting region. The light-emitting region has a structure in which an anode, an organic emission layer, and a cathode are sequentially stacked on a substrate, and the non-light emitting region has a structure in which a thin film transistor (TFT) and metal wires are formed. The cathode of the light-emitting region, and the TFT and metal wires of the non-light emitting region have high reflectability, and thus, as external light becomes brighter, the reflection of external light increases. Consequently, a reflected image overlaps with a desired image, and thus it is difficult to realize clear images.
Such reflection of external light may be quantitatively evaluated by measuring ambient contrast ratio (ACR). ACR is defined by the equation below, and the black luminance (Lblack(off-state)), of an organic light-emitting device is approximated to be 0, thereby inducing a simple formula:
                    ACR        =                ⁢                              (                                          L                                  white                  ⁡                                      (                                          on                      ⁢                                              -                                            ⁢                      state                                        )                                                              +                              R                ×                                  L                  ambient                                                      )                    /                      (                                          L                                  black                  ⁡                                      (                                          off                      ⁢                                              -                                            ⁢                      state                                        )                                                              +                              R                ×                                  L                  ambient                                                      )                                                  ≈                ⁢                  1          +                                    (                              L                                  white                  ⁡                                      (                                          on                      ⁢                                              -                                            ⁢                      state                                        )                                                              )                        /                          (                              R                ×                                  L                  ambient                                            )                                                              ≈                ⁢                  1          +                      (                                          luminance                /                reflected                            ⁢                                                          ⁢              luminance                        )                              
It is assumed that an external light source is Lambertian-reflected from a surface of an organic light-emitting display device. The illuminance (LAmbient, lux) of the external light source is converted to luminance (cd/m2). When the luminance (LWhite(on-state)) of the organic light-emitting display device is 200 nit and the reflectance of an organic light-emitting diode is 5% at 150 lux, which is illumination of indoor lamps, ACR is calculated to be about 85. This ACR value is a satisfactory value, and thus there are no problems in displaying images. However, an average reflectance by reflection of a cathode, and by reflection of TFTs and/or metal wires of a general organic light-emitting display device, is about 65%. Thus, when an aspect ratio of a light-emitting region is about 30% and the reflectance thereof is about 60%, and an aspect ratio of a TFT/metal wire region is 70% and the reflectance thereof is about 70%, the average reflectance is 67%. When the average reflectance is about 65%, ACR is calculated to be about 19 at a luminance of 550 nit and an illumination of 150 lux, and an actual measurement value thereof is about 14. Under these conditions, it is difficult to properly view images displayed in an organic light-emitting display device.
To address these problems due to the reflection of external light, a circularly polarized film (e.g., hard coat surface-treated circularly polarized film) or a color filter (e.g., an organic light-emitting diode having a resonance structure and a color filter) is used to reduce the reflectance of an organic light-emitting display device to about 5% to 7%. However, the use of the circularly polarized film requires a protective film, a linear polarized film, and a λ/4 phase difference film, and at least two sheets of protective films and phase difference films are used. In addition, the transmittance of light by an absorption-type linearly polarized film is less than 50%, and thus the luminous efficiency of an organic light-emitting display device is reduced. Also, light of an organic light-emitting display device is polarized light, and thus, when three-dimensional (3D) images are viewed in a passive glasses manner, transmittance characteristics due to viewing angles may be deteriorated. On the other hand, when manufacturing a bottom-emission type organic light-emitting display device and when the color filter is used, a black matrix and the color filter need manufacturing prior to a manufacturing process of a TFT, and thus the manufacturing processes are more complicated than when the circularly polarized film is used. In addition, in the bottom-emission type organic light-emitting display device, when Cr/CrOx or an organic CF material is subjected to a high-temperature manufacturing process of a TFT, either the properties of the material are deteriorated or the material may act as an outgassing source.